In the production of oil and gas, chemicals such as corrosion inhibitors, scale inhibitors, paraffin inhibitors, hydrate inhibitors, and demulsifiers are typically injected into the wells to maintain efficient flow of oil or gas. These chemicals usually need to be added to the wells production at a constant rate. Often one pump is used to inject the same chemical into several wells with the use of pressure compensated rate control valves at each injection point. The use of these rate controllers reduces set up and operating costs of injection systems because the alternative is to install a separate pump for each injection point and to maintain several pumps instead of one. These injection valves must be pressure compensated because they need to maintain a rate set point with changes of several thousand pounds per square inch across them to accommodate fluctuations in well pressure. A typical chemical injection rate for an oil well is between 0.5 to 200 US gallons per day. Injection pressures range between 500 to 20,000 psi.
A conventional method to achieve rates in this range using pressure compensated rate controllers is to govern the pressure drop across a fixed orifice. The set point for this method is changed by varying the pressure drop across the orifice. This method is described in U.S. Pat. No. 4,893,649. Previous methods to vary the area while maintaining a constant pressure drop have not adequately worked in the low flow range because passages created by mating needles and trims or mating threads to restrict flow are often less than 0.001 inches wide, which makes them prone to clogging and/or filming. The fixed orifice method is robust since hole passage can be made to pass the largest debris for a given flow area and several holes cascading in series can be used to give the same resistance with as much as a twenty fold increase in the flow area reducing the filming and clogging tendencies. The consequences of varying the pressure drop across a fixed resistor is that the range of flow rate set point is limited and passages cannot be opened up to pass blockages as can be done with a mating needle and trim.
Set point range of a valve is defined by its “turn down,” which equals the valve's highest flow rate divided by the lowest flow rate achievable. For a fixed valve orifice, the turn down is calculated by taking the square root of the highest pressure drop across the orifice divided by the lowest pressure drop. For example, a valve that offers a pressure drop across the orifice of 200 psi at maximum flow and 2 psi at minimum flow will have a turn down of 10:1. During the life of the well the flow rate range may need to be adjusted, which involves replacing an orifice. Sending personnel or equipment to remote locations to change an orifice represents a substantial expense, particularly if the valve location is under water. U.S. Pat. No. 7,770,595, which is incorporated herein in its entirety by reference thereto, provides a very effective and improved constant flow control valve with a very wide range of flow rate set points independent of pressure changes across the valve.
Pressure compensated flow control valves are designed to maintain constant flow with changes in pressure drop across the device, wherein the flow passes to the underside of a throttling member, such as a mating cone and sharp edged seat (U.S. Pat. No. 6,662,823) and a sharp edged hollow cylinder (U.S. Pat. Nos. 4,250,915 and 5,642,752). In these flow control valves, the flow path is, as an example, under the throttle cone first and then through the mating seat, such that the valves are susceptible to inadequate control or inadvertent closure of the throttle upon a large pressure differential or a pressure spike in the fluid entering the valve. Accordingly, these pressure compensated valves are typically not designed to adequately handle large pressure drops across the valves.
U.S. Pat. Nos. 6,827,100 and 4,210,171 discloses control valves with fluid flow going under the seat first. These control valves, however, are not adequately balanced to handle large pressure drops across the valves or large, sudden pressure spikes (i.e., transient pressure spikes). As a result, the balance of these valves will become unstable with pressure spikes or large pressure drops across the valves.
Pressure balanced rate control valves, such as those disclosed in U.S. Pat. No. 4,893,649, Skoglund U.S. Pat. Nos. 5,234,025, and 6,932,107 are unique from other prior art pressure compensated rate controls because the ratio of the area balanced by the spring chamber is substantially larger than the area of the seat that dissipates the pressure drop. These pressure balanced rate control valves, however, have a configuration and flow direction such that the valves can go into a cyclic opening and closing sequence (sort of an on/off water hammer) with excessive pressure drops across the valve. This cyclic opening and closing can provide an undesirable harmonic cycling that will match the natural frequency of the piping supplying the valve.
A significant problem for conventional flow rate controllers is cavitation. Cavitation will typically occur in a valve trim if the fluid velocities are fast enough to cause the pressure at the velocity point to drop below the vapor pressure of the liquid. When pressure is dropped below vapor pressure it will create a collapsing bubble when the pressure is recovered. In short, such cavitation occurs when the pressure drop across the valve is greater than the valve's outlet pressure (subject to correction for the vapor pressure). The resulting collapsing bubble causes a point pressure load of up to 300,000 psi pressure on valve surfaces. This high contact pressure also causes an instantaneous heating at the collapsing bubble. The high heat and high contact pressure will erode the surfaces and will generate high frequency flow noises reverberating to the piping system. Cavitation and trim selection to avoid cavitation is described in ANSI/ISA-75.01-2002 “Flow Equations for Sizing Control Valves.” There is a need for a flow rate controller that effectively eliminates cavitation across its operating conditions.
Back pressure regulators have also been commercially available for decades to help serve as a relief valve or constant spill off device to limit excess pressure to a desired operating pressure range. A multi-stage back pressure regulator, as disclosed in U.S. Pat. No. 9,122,282, which is incorporated herein in its entirety by reference, provides a significant improvement to the back pressure regulator technologies. Conventional back pressure regulators, however, also suffer from cavitation conditions that can occur when there is a large pressure differential (e.g., 1,000 pounds per square inch (psi)) between the inlet and the outlet.
High pressure differential can also cause other problems, such as high frequency flow noises that reverberate throughout a piping system. These noises can be extremely loud and may, in some cases, require installing noise suppression systems to meet safety standards. Another problem with conventional back pressure regulators is that they can include internal components that work against one another. For example, U.S. Pat. No. 8,375,983 discloses a two stage device with the second stage governing the pressure drop across the first stage. The second stage balances two different pressures inside the regulator against the pressure outside of the regulator over a bound area to create a force that governs the pressure differential across the first stage. A spring governs the pressure drop across the stage upstream of it. Flow passes through the second stage by going around a throttling pin then through the throttling seat. In this arrangement, a spike in inlet pressure will cause the second stage piston to drive towards the seat causing unstable pressure regulation. When two or more of these devices are installed in parallel, they can fight each other without external pressure spikes causing this effect.
There is a need for flow control valve assemblies and pressure regulator assemblies that can operate under very high pressure differentials without causing cavitation and excessive reverberation.